sched/topology: fix KASAN warning in hop_cmp()

Despite that prev_hop is used conditionally on cur_hop
is not the first hop, it's initialized unconditionally.

Because initialization implies dereferencing, it might happen
that the code dereferences uninitialized memory, which has been
spotted by KASAN. Fix it by reorganizing hop_cmp() logic.
Reported-by: Bruno Goncalves <bgoncalv@redhat.com>
Fixes: cd7f5535 ("sched: add sched_numa_find_nth_cpu()")
Signed-off-by: Yury Norov <yury.norov@gmail.com>
Link: https://lore.kernel.org/r/Y+7avK6V9SyAWsXi@yury-laptop/Signed-off-by: Jakub Kicinski <kuba@kernel.org>

kernel/sched/topology.c

@@ -2079,14 +2079,19 @@ struct __cmp_key {
 
 static int hop_cmp(const void *a, const void *b)
 {
-	struct cpumask **prev_hop = *((struct cpumask ***)b - 1);
-	struct cpumask **cur_hop = *(struct cpumask ***)b;
+	struct cpumask **prev_hop, **cur_hop = *(struct cpumask ***)b;
 	struct __cmp_key *k = (struct __cmp_key *)a;
 
 	if (cpumask_weight_and(k->cpus, cur_hop[k->node]) <= k->cpu)
 		return 1;
 
-	k->w = (b == k->masks) ? 0 : cpumask_weight_and(k->cpus, prev_hop[k->node]);
+	if (b == k->masks) {
+		k->w = 0;
+		return 0;
+	}
+
+	prev_hop = *((struct cpumask ***)b - 1);
+	k->w = cpumask_weight_and(k->cpus, prev_hop[k->node]);
 	if (k->w <= k->cpu)
 		return 0;